1. Field of the Invention
The present invention relates generally to Code Division Multiple Access communications. Particularly, the present invention relates to time tracking received signals in a non-negligible multipath spacing environment.
2. Description of the Related Art
Code Division Multiple Access (CDMA) communications systems use base stations coupled to directional antennas that are typically located in the center of a cell and broadcast into sectors of the cell. The cells are located in major metropolitan areas, along highways, and along train tracks to allow consumers to communicate both at home and while traveling.
FIG. 1 illustrates a block diagram of a typical CDMA base station. The user data is input to a modulator (101) that performs the CDMA modulation prior to transmission on the single antenna (105). The CDMA modulation technique is well known in the art.
The base station transmits a pilot channel that is received by a mobile station. The pilot channel, comprised of symbols, contains no information. The mobile station uses the pilot channel as a reference signal for time, frequency, phase, and signal strength.
Mobile stations are comprised of RAKE receivers. A conventional RAKE receiver operates on received signals with correlators known as “fingers”. Using the knowledge of complex channel coefficients of each desired multipath component, the RAKE receiver coherently combines finger outputs.
A block diagram of a typical RAKE receiver is illustrated in FIG. 2. For purposes of clarity, only one finger of the receiver is shown. The receiver is comprised of an antenna (201) that receives the signal for conversion from the received radio frequency-to-baseband frequency (205). The base band data is a digital data stream.
An initial time delay, τ, is chosen (210) and the digital data stream is despread by multiplying (215) it with the original spreading sequence combined with a Walsh code. This is referred to as cd(n).
The despread signal is correlated by summing (220) it over a symbol time (64 chips). The complex signal output from the correlator is multiplied (225) with an estimate of the pilot signal, {circumflex over (P)}, in order to rotate the phase of the input signal. This step outputs the demodulated data.
In parallel with the demodulation at time τ, the digital data stream is also demodulated half a chip prior to τ and half a chip after τ in order to generate a more accurate τ. It would be best to sample the waveform at the peak during the on-time sample. However, since this cannot always be accomplished, an early sample is taken approximately half a chip time before the on-time sample and a late sample is taken approximately half a chip time after the on-time sample.
After the ±0.5 chip delay blocks (230 and 235), the delayed digital data streams are multiplied (240 and 245) with a combination of the same spreading sequence used in the demodulation path and the pilot Walsh code. This is referred to as cp(n). These signals are correlated (250 and 255) and the magnitude of each signal is then squared (260 and 265).
The squared magnitudes are subtracted (270) to find the difference between the two energies. If the difference is zero, the initial estimate for τ was accurate. If the difference is other than zero, this error is input to a time tracking loop (275) to generate a new τ estimate τ. Each finger tracks its assigned signal path using the time tracking loop (275) by controlling the finger's location with respect to time.
The above-described receiver performs adequate time tracking if the single base station antenna of FIG. 1 is used. However, if the base station uses antenna diversity, as illustrated in FIG. 3, time tracking becomes more complex in a non-negligible multipath spacing environment.
FIG. 3 illustrates a typical prior art base station where the main data signal is input to a multiplexer (301) before being modulated (305 and 310). The multiplexer (301) switches the data between two or more modulation paths (305 and 310). Each modulation path (305 and 310) is coupled to a separate antenna (315 and 320).
The antennas are typically geographically separated such that the received signal at the mobile station has approximately the same time delay with independent fading characteristics. The most common methods for multiplexing data at the base station are Orthogonal Transmit Diversity and Space Time Spreading.
The Orthogonal Transmit Diversity scheme alternates the transmitted data between the transmit antennas such that each antenna is transmitting a different data signal that is a subset of the main data signal. For example, a first symbol of the main data signal is transmitted on the first antenna, a second symbol is transmitted on the second antenna, and a third symbol is transmitted on the first antenna. In this manner, if the mobile station loses data from one of the antennas, it only loses every other symbol and an error correction routine can correct for the loss.
The Space Time Spreading scheme transmits some information about each data symbol on both antennas. This scheme assumes that the mobile station will be in contact with at least one of the antennas at all times and, therefore, will continue to receive uninterrupted data.
A problem occurs when a mobile station's receiver has to time track on the signals from both base station transmit antennas, and the multipath spacing from one or both of these antennas is non-negligible (e.g., multipath spacing is less than 1.5 chips). There is a resulting need for a receiver that can time track in a non-negligible multi-path environment having antenna diversity.